氧化石墨烯的变温发光

石墨烯的性能优异,在电子器件、导热材料和复合材料等领域有潜在的应用价值[1~6] 因此,近年来关于石墨烯材料的研究受到了高度重视 但是,石墨烯是零带隙半导体[6~8],没有发光特性 氧化石墨烯(Graphite oxide,GO)是石墨烯重要的衍生物之一,是规模化生产石墨烯的原料 GO和石墨烯的结构差异很大,GO内部有羟基、羧基和环氧基等大量氧化官能团 氧化官能团破坏了石墨烯片层的 π 共轭体系,使其电学性质和光学性质发生了巨大变化,由导电（石墨烯）变为绝缘(GO)[9~12]并具有光催化活性[13~15] 特别是sp2C/sp3C的交替分布打开了石墨烯的带隙,使其具有发光性能 GO发光分布在可见光和近红外波段,可用于生物检测[16,17]和荧光标记[18] 目前对氧化石墨烯光学性质的研究刚刚展开,对其能带结构的认识和发光机理的理解还很不深入 本文根据光致发光光谱、变温发光光谱和吸收光谱,研究GO的发光机制和不同激发波长与变温条件下的发光光谱,以揭示不同局域态的发光行为

1 实验方法1.1 实验用材料和仪器

天然鳞片石墨(325目)；微孔滤膜(醋酸纤维酯,直径50 mm,孔径0.22 μm)

inVia型发光光谱仪(PL)；Lambda900型紫外-可见吸收光谱仪(UV-Vis)

1.2 氧化石墨烯的制备

用改进的Hummers法[19],将天然鳞片石墨通过超声辅助液相氧化法制备氧化石墨烯（GO） 用真空抽滤法制备GO薄膜,改变过滤GO溶液的量或浓度,调节薄膜的厚度 分别在488 nm、514 nm和830 nm激发条件下测试GO薄膜的荧光光谱 在514 nm和830 nm激发条件下研究GO薄膜的原位变温发光

2 结果和讨论2.1 氧化石墨的吸收特性

图1a给出了GO的紫外-可见吸收光谱 可以看出,吸收谱有227 nm和300 nm两个峰,分别来自于C=C键的π-π*电子跃迁和C=O键的n-π*电子跃迁[20,21] 在吸收光谱上没有发现清晰可辨的吸收边,表明GO内分布着很多局域态[22] 在强吸收区α≥104 cm-1范围(Tauc region),吸收系数与光学带隙之间满足Tauc方程

(αE)1/2=B(E-Eopt)

(1)

其中B为常数,与材料性质有关[23]；E为光子能量；Eopt为光学带隙[24~26],拟合可得到GO的Eopt=1.58 eV 在弱吸收区2×102 cm-1<α<5×103 cm-1范围内(Urbach tail)[27],吸收系数和E满足e指数关系

α=α0exp(E/E0)

(2)

其中E0为urbach能,表征带尾态的宽度,与材料的无序程度相关 根据 式(2)拟合得到E0=1.06 eV,表明GO具有较高的无序度且有较广的局域态

图1



图1GO的紫外-可见吸收光谱和GO吸收光谱的Tauc拟合图

Fig.1UV-Vis absorption spectra of GO (a) and corresponding Tauc plot (b)

2.2 常温条件下GO的发光特性

图2给出了GO分别在Eex =488 nm(2.53 eV>Eopt),Eex=514 nm(2.41 eV>Eopt)和Eex =830 nm(1.49 eV<Eopt)激发条件下的光致发光光谱(PL) GO的PL发射峰都具有非常宽的谱带；随着激发光能量的降低发射峰中心波长逐渐红移,半峰宽(FWHM)变窄

图2



图2GO在488 nm,514 nm和830 nm激发波长下的发光光谱和GO 发射峰的中心波长(左)和半峰宽（右）与激发光波长的关

Fig.2PL spectra of GO for excitation at Eex =488 nm, Eex =514 nm and Eex =830 nm (a) and the dependences as the function of excitation wavelengths: (left) peak wavelength and (right) FWHM of PL spectra (b)

石墨烯是一种零带隙半导体,无发光特性[26~28] 但是当其尺寸减小至纳米级别时,由于量子限域效应带隙被打开,具有发光特性[28] GO的发光来自于量子限域效应,其发光中心由小尺寸的sp2C团簇组成 sp2C团簇的π-π*间的带隙被高势垒的氧化官能团(sp3C区域)包围形成量子阱结构,如图3所示,此能带结构与非晶碳相似[30]

图3



图3非晶碳的能带结构

Fig.3Schematic diagram of band structure of GO

用多量子阱的能带结构可解释GO的发光特征 sp2C团簇π-π*间的带隙与尺寸相关,碳团簇的尺寸越小带隙越宽 GO的片层结构中存在不同尺寸的sp2C区域,因此分布着非常多的局域状态,可由吸收光谱中较大的urbach能证明 由于共振吸收效应,激发光能量与发光中心带隙能量相同时吸收最强 在不同激发条件下,因共振吸收效应参与的发光中心不同,因此随着激发能量降低发光的主峰位置红移 同时,随着激发能量的降低可被激发的发光中心减少,使发光峰的半峰宽变窄

2.3 GO的变温发光性质

图4给出了GO发光强度与温度的关系 在514.5 nm和830 nm激发条件下和80 K~300 K范围内,GO的发光强度降低一个数量级,强度变化的转折点分别出现在220 K(Eex =514.5 nm)和160 K(Eex =830 nm) GO的发光强度随温度的变化在一个数量级内,与非晶碳发光随温度的变化关系相似[23,30,31] 以往的研究表明,在非晶碳材料体系中,即使具有较高的缺陷密度(发光猝灭中心密度大),温度对发光强度的影响仍然较小[31] 其原因是,电子-空穴对被sp3C区域的高势垒限制在局域的碳团簇中,使电子-空穴的波函数交叠变大,大大提高了发生辐射跃迁的几率[30,32] 相反,sp3C势垒层被破坏后,由于体系缺陷的密度较高,在声子辅助下电子快速转移缺陷处复合,在室温下很难观察到发光现象,如a-Si1-x Cx:H(x<0.09)[32] 制备过程中的强氧化作用,使GO中存在大量缺陷态(如碳空位) 但是,高势垒sp3C区域对sp2C团簇的限域效应,限制了电子到缺陷态的复合,使辐射跃迁几率提高,在室温下即可观察到发光现象 氧化官能团(高势垒区)被破坏后,如氧化石墨烯被还原,虽然GO内仍然存在小尺寸的sp2C团簇(发光中心)仍然能观察到发光现象[33]

图4



图4两种不同激发条件下GO相对发光强度的温度依赖性：(a) Eex =514 nm; (b) Eex =830 nm

Fig.4Temperature dependence of the relative PL intensity of GO at two different exciting conditions: (a) Eex =514 nm; (b) Eex =830 nm

在不同激发波长条件下,变温发光随着温度的变化呈现不同的变化趋势 在514 nm激发下在80~220 K,GO的发光强度受温度的影响较小,温度≥220 K时发光强度明显降低；在830 nm激发下在80~300 K,GO的发光强度随着温度的降低而降低,温度≥160 K时发光强度降低的速率变大 利用Arrhenius方程[34],可拟合得到发光猝灭的热激活能

IPL(T)=I0/(1+Aexp(-Ea/kBT))

(3)

其中T为温度,kB为波尔兹曼常数,I0为0 K附近的PL谱积分强度,A为常数,Ea 为热激活能 拟合结果为：Eex =514 nm,热激活能Ea =119 MeV(图5a)；Eex =830 nm,热激活能Ea =63 MeV(图5b) 这表明,与Eex =830 nm相比,在Eex =514 nm激发下参与发光的sp2C团簇的热稳定性更高,发生温度猝灭所需的热激活能高达56 MeV 根据GO的多量子阱结构,可解释不同激发波长下的变温发光 由于共振效应,不同能量的光激发不同尺寸的局域态(sp2C团簇) 与Eex =830 nm相比,Eex =514 nm激发的sp2C团簇尺寸较小 在多量子阱结构中,由于量子尺寸效应,sp2C团簇尺寸越小能级间隔越大,热猝灭需要的参与的声子数目增加,因此尺寸越小发光强度受温度的影响越小

图5



图5光致发光强度与1/KBT之间的关系

Fig.5Relationship between relative PL intensity and 1/KBT, exciting at Eex =514 nm (a) and Eex =830 nm (b)

3 结论

用多量子阱结构能解释GO的发光特性 GO的发光来自于片层内的sp2C团簇 sp2C团簇被高势垒的氧化官能团（sp3C）包围,形成了多量子阱结构 GO内有不同尺寸的sp2C团簇,由于量子尺寸效应sp2C团簇的带隙与尺寸相关 因此GO的发光呈现出对激发波长的依赖,尺寸越小带隙越宽,温度对发光强度的影响越小

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